Epinephrine
Epinephrine. Endogenous catecholamine and emergency drug. Mixed α1/α2/β1/β2/β3 agonist (with higher β-affinity than norepinephrine due to the… IV half-life 0.1 h.
Endogenous catecholamine and emergency drug. Mixed α1/α2/β1/β2/β3 agonist (with higher β-affinity than norepinephrine due to the N-methyl group added by PNMT). Endogenously synthesized in adrenal medulla + brainstem PNMT-expressing neurons under sympathetic activation. Clinical use: IM anaphylaxis (0.3-0.5 mg adult / 0.15 mg pediatric), ACLS cardiac arrest (1 mg IV q3-5 min), septic shock vasopressor, local-anesthetic vasoconstrictor (with lidocaine to extend block + reduce systemic absorption), nebulized for upper-airway croup. Extremely short t½ (~2-3 min IV) due to rapid MAO/COMT degradation — bolus or infusion only. PK (Coté 2009 critical-care popPK PMID:19534820 + EpiPen label): IV anaphylaxis/cardiac-arrest workhorse + IM autoinjector for outpatient anaphylaxis. t½ ~3.5 min — among the shortest of all clinical drugs. Vd 8 L (vascular + minimal tissue). Clearance dominated by hepatic MAO + COMT extraneuronal metabolism (cross-link: monoamine_oxidase_metabolism + catecholamine_synthesis). Oral F essentially zero (massive first-pass MAO/COMT). IM/SC absorption via local vasoconstriction is paradoxically variable; IM thigh (vastus lateralis) preferred over deltoid for autoinjector use (faster + more reliable absorption).
ROUTES REORDERED so the dosing default is a solvable route. IM stays declared and is the emergency route, but the EpiPen label states no bioavailability, volume or half-life, and IM epinephrine PK is dominated by local vasoconstriction at the injection site, which a first-order ka cannot represent. IV is authored and becomes the default. PK: MY HOMEOSTATIC PRIOR IS REFUTED — THE CITATION ARGUES THE OPPOSITE, VERBATIM. I expected an endogenous catecholamine with a strong basal concentration to belong with the minerals in the unauthored class. Its source concludes "Epinephrine pharmacokinetics is linear in septic shock patients, without any saturation at high doses" and explicitly checked the basal question: "Basal neurohormonal status does not influence epinephrine pharmacokinetics." A dosed first-order curve is the right object here, and the record keeps it. The defect is smaller and ordinary: that paper states its clearance as an equation, "CL (L/hr) = 127 x (BW/70)0.60 x (SAPS II/50)-0.67", which at this catalogue seventy-kilogram reference and the score the equation is centred on is 127 L/h, against a stored assertion of 96 — a third too slow. The half-life is verbatim, "The corresponding half-life was 3.5 minutes", so the volume moves to the one that clearance implies at that rate. TWO CAVEATS TRAVEL WITH IT: the cohort is septic-shock patients under steady-state infusion, not healthy volunteers, and an intramuscular route is declared with a dose but NO parameters, so the autoinjector dose the record carries yields no curve at all. PK: THE INTRAMUSCULAR ROUTE IS THIS RECORD DEFAULT AND ITS COMMONEST REAL USE — the anaphylaxis autoinjector — AND IT CANNOT BE MODELLED HERE. That is a finding, not an omission, and the model proves it. A one-compartment peak time is bounded, because Tmax = ln(ka/ke)/(ka-ke) rises as ka approaches ke and cannot exceed 1/ke; at this record verbatim intravenous half-life of 3.5 minutes that CEILING IS 5.0 MINUTES. The published intramuscular peak is "Tmax 5-10 min" — at the ceiling at best and past it at worst — SO NO ABSORPTION RATE REPRODUCES IT. The reason is flip-flop kinetics: intramuscular absorption is slower than elimination, so this route apparent terminal slope reflects ABSORPTION rather than elimination and cannot inherit the intravenous half-life. Authoring it needs an intramuscular-specific apparent half-life and no indexed abstract states one. AND THE PROFILE IS BIPHASIC REGARDLESS: a systematic review of the intramuscular literature reports verbatim that "Most investigators found two Cmax's with Tmax 5-10 min and 30-50 min, respectively", while a single first-order rate produces exactly one peak — so even with a correct half-life this route would be structurally misrepresented. Recorded before someone fits a rate to the first peak and calls the record finished.
Pharmacokinetics
- IV — t½ 3 min · F 100% · Vd 10.69 L
Sources: PMID:19622169, PMID:33685510, PMID:37604314